The big scary thing right away is that it stores the data in a cloud. It also connects directly to my bank account.
If I'm storing data about payments, people, and things, then I want to keep this data safe. I'm actually thinking about storing this data on a computer that doesn't access the internet. But... when I talk to the bank, they don't have any concerns about online data - they feel that clouds are very safe. I guess they don't have friends who are hired by the government to hack clouds - for research purposes so they can make government clouds more secure. I don't understand it all, but what I get from those conversations is that clouds are not safe places for data yet.
I use Wave for my small business (two rental units) and I enjoy it. I do not have it linked to any bank accounts and instead enter all the details by hand. So, the dollar amounts and scanned receipts live in the cloud, but no bank account numbers or any other such thing.
I don't keep any inventory, so I'm not sure how well that would work.
Every year, I just export a profit/loss statement along with the associated receipts and send that along to my accountant for taxes. It is quite streamlined from my point of view.